Natural Selection may take a variety of
forms and act on any behavioral, morphological, developmental or physiological
traits of an organism.However, certain
types of selection are unique in their features, and they are often treated as
special categories of selection.One of
these "special" categories is that of sexualselection.

Darwin was the first to realize the
existence and importance of sexual selection, which he defined as "the
advantage which certain individuals have over others of the same sex and
species solely in respect of reproduction".The modern concept of sexual selection is usually restricted to
characteristics which affect mating
success, not reproduction in general, but most organisms must mate in order to
reproduce!

Darwin (1871) developed the notion of
sexual selection because he realized that not all of the differences between
males and females were due to their specialized roles in sexual
reproduction.Features found in males
of the species such as horns or antlers (or disproportionately larger ones) in
ungulates (e. g., deer, gazelles), significantly larger body size in many
mammals, or the bright plumage common in birds have nothing to do with sperm
production or the physical aspects of mating.In fact, Darwin realized, some characteristics, like bright plumage,
might be detrimental to an individual because they hampered the organism or
made it more susceptible to predation.However, such characteristics would be favored if the enhancement of
mating success outweighed (or at least equaled) their detrimental effects.

Sexual Selection may take two forms: (1) Intrasexual Selection involves
characteristics which affect the outcome of competition among members of one
sex for access to members of the other sex.For example, intrasexual selection would operate on physical and
behavioral features which helped to determine the outcome of aggressive encounters
among males over territories, if possession of or the quality of a territory
affected subsequent mating success.(2)
Intersexual Selection would
influence the evolution of secondary sexual characteristics which determine the
relative "attractiveness" of members of one sex to the other
sex.Such items as courtship displays
and male plumage in birds (e. g., the male peacock) are obvious examples, but
the whole phenomenon of mate choice
may often be subtle (see Bateson, 1983).Often, it is difficult to separate the two forms of sexual
selection.The earlier case in which
males competed among themselves for high quality territories, which would then
determine the number of females who mated with each male, is clearly a case
where there are components of both intrasexual and intersexual selection.

Sexual Selection is also noteworthy
because it has the potential to become a "runaway" process in which
there is no obvious optimum set of characters.In other categories of selection there is at least the potential for
some defined equilibrium point.However, in sexual selection there is no optimum as long as the
potential exists for further improvements in mating success.Take the hypothetical case where a female
bird chooses her mate based on some plumage characteristic like tail feather
length.The subsequent male offspring
will inherit the tail feather length, but the female offspring will inherit the
mate preference behavior so that the cycle of selection will continue in the
next generation.The process of sexual
selection has the potential to become a case of positive feedback in which the characteristic is continually
enhanced in an (almost) endless cycle.

R. A. Fisher was the first to define the
possibility that sexual selection could become a "runaway"
process.However, he did not examine
this process in explicit, quantitative genetic detail.Recent models by Russell Lande have shown
that the runaway process is not inevitable (see review by Arnold, 1987).According to Lande's work, the runaway
process will be most likely to occur if: (1) there is a high genetic
correlation between male and female traits (i. e., a high genetic covariance);
(2) there is weak natural selection on the male characters; (3) there is a
strong mating preference by the females.The runaway process is in fact an example of an unstable equilibrium,
but the stable case, which occurs when there is weak intersexual correlation,
strong natural selection, and a "weak" mating preference, has some
interesting properties.In some models,
the stable equilibrium is not a single point on the adaptive topography, but
instead it is a line of equilibrium
values.In other words, it represents a
neutral equilibrium, with the
population's final location being affected by other factors - such as genetic
drift (see figure).However, regardless
of the nature of the equilibrium, most models show that the population may
reach an equilibrium below the peak of the adaptive topography because the
balance between sexual selection and other selective processes can reduce the
average fitness of the population.

Regardless of the form of equilibrium,
the process of sexual selection is limited.Sexual selection will cease to affect a character when one or more of
the following occur: (1) There is no longer any genetic variation for the
characteristic; all individuals are both genotypically and phenotypically
identical. (2) There is still genetic variability, but it results in phenotypic
differences which cannot be perceived by other individuals, and some other
characteristic becomes the basis of sexual selection. (3) The effects of sexual
selection are counterbalanced by some other form of selection.For example, there may be plenty of
potential for brighter male plumage, but any increase in male "showiness"
would raise the risk of predation to the point where a male would be unlikely
to survive reaching sexual maturity.

Sexual selection usually operates on the
sex with the greater degree of variability in mating success.Most of the documented cases of sexual
selection focus on males because, presumably, all females will be mated, but
males will vary in their mating success.This is especially true in polygynous
species where a male may control mating with a group of females (sometimes such
males are described as having a harem), and so some males make a
disproportionate contribution to the next generation, while some males do not
mate at all.However, there are cases
in which sexual selection acts on females because the males make a reproductive
contribution which varies so that some males are more "desirable"
mates than others (e. g., Petrie 1983, 1986), and there is female competition
for these males.

The discussion so far has avoided the
issue of why sexual selection for a particular character should exist at
all.While intrasexual selection may be
easily understood and observed, the phenomenon of intersexual selection is more
subtle because it is not clear why, or how, a particular feature became the
basis by which mates are chosen.That
there is a genetic basis to mate preferences has been demonstrated in a number
of cases (Majerus, 1986).Less certain,
however, is if there is any genetic correlation between the sexually selected
trait and any component of mate fitness, or fitness of the subsequent
offspring.In fact, some have argued
that the sexually selected trait has its own intrinsic value because of the
enhanced mating success of the offspring, but the so-called "sexy
son" hypothesis does not hold up under quantitative genetic scrutiny
(Arnold, 1987).

The fact remains that we have few good
examples of the "good genes" hypothesis, which holds that there is a
correlation between the intersexually selected trait and the fitness of
subsequent progeny.One hypothesis, by
Hamilton and Zuk (1982), proposes that sexually selected characters are
correlated with resistance to parasites.This hypothesis is based on the fact that parasitism is a prevalent,
powerful selective agent, and if resistance to parasites is a heritable
characteristic, correlated with a qualitative secondary sexual characteristic
(e. g., plumage color), then this could be a case that fulfills the "good
genes" scenario.Although there
are specific requirements associated with the Hamilton and Zuk hypothesis,
there is some empirical evidence to support the role of parasitism in sexual
selection. (Read, 1988).

Sexual selection may also play a role in
the process of speciation because mate preferences are obvious foundations for
premating reproductive isolating mechanisms.Russell Lande (1981, 1982) has investigated this possibility.Given the possibility that sexual selection
can result in linear, rather than point equilibria, it is easy to imagine two
populations which have the same intersexual system, but they attain different equilibria
as a result of genetic drift and/or slight differences in initial
conditions.Presumably, individuals
moving from one population to the other would experience reduced fitness due to
decreased mating success.Lande (1982)
has also shown that sexual selection may exaggerate clinal variation, which
would further increase the likelihood of speciation.

The most intriguing hypothesis to develop
from recent discussions of the importance of sexual selection is William
Eberhard's proposal regarding the evolution of animal genitalia (Eberhard,
1985, 1990).In many species of
animals, especially insects, external genital structures are both elaborate and
species specific.The classical
explanation is that these structures act as pre-mating reproductive isolating
mechanisms - the "lock and key" hypothesis - requiring proper
matching for the transfer of gametes.However, precise physical matching of genitalia is not necessary for
successful gamete transfer (Eberhard, 1990).Instead, he argues that the external genitalia are part of an elaborate
premating system that is necessary to guarantee fertilization.In other words, the genital structures have
evolved through a subtle form of sexual selection.This means that the potential range of sexual selection should be
extended to many, if not all, prezygotic phenomena.